A President, a Speech and a Point of Law

In the speech, Mr. Obama said that one of the people killed on Saturday, John M. Roll, the chief federal judge in Arizona, “was on his way back from attending Mass, as he did every day, when he decided to stop by and say ‘Hi’ to his representative” — a characterization that suggested he was a bystander when the shooting began.

But the criminal complaint against Mr. Loughner says Judge Roll “was engaged in performance of official duties,” a critical element of the 1971 law that designated the killing of federal employees as a federal crime.

Legal experts said the question in prosecuting Judge Roll’s death as a federal crime would be what the evidence showed about why he was going to see Ms. Giffords — including whether he had stopped to talk about their work in seeking additional resources for Arizona’s federal courts. If the prosecution of Judge Roll’s shooting does not pass muster as a federal case, then state charges will apply.

Rory Little, a former Justice Department official in the Clinton administration who now teaches at the University of California Hastings College of the Law in San Francisco, said, “Based on what we know, it seems like a difficult issue, and importantly, not an issue that would arise in a state prosecution.”

While the debate shows the sensitivity involved when presidents talk about criminal cases — Charles Manson’s lawyers moved for a mistrial in 1970 when President Richard M. Nixon weighed in about his guilt — it is unlikely that Mr. Obama’s comments will have much of an effect at trial, said John C. Coffee Jr., a professor at Columbia’s law school.

“I am not certain that the president’s statement is even admissible,” he said. And even if the defense is able to introduce it, “it is not irrefutable evidence, and it does not ‘estop’ the government” — meaning to preclude the government from offering evidence to the contrary.

In any case, the federal charges are only part of the complex legal landscape in the Tucson case, with a state trial in the offing.

Andrew C. McCarthy, a former federal prosecutor and a critic of the Obama administration, said that the president’s characterization of events “certainly doesn’t help matters,” and could prove to be “a minor boon to the defense.” But, he added, the remarks were made “for an appropriate civic purpose and don’t deny anyone a fair trial.” And so, he said, “I would cut the president some slack on this one.”